This invention relates to masks used in lithography and, more particularly, to a method of making a mask which can be used for X-ray lithography.
In the manufacture of integrated semiconductor circuits, various types of lithography are used in order to expose a photoresist which is deposited on the semiconductor wafer. With requirements for miniaturization and placing more devices on a given chip, lithography has progressed towards shorter wavelengths. Shorter wavelengths are necessary in order to obtain good resolution and small size. Thus, there has been a progression from optical lithography utilizing the visible spectrum to ultraviolet lithography and, more recently, to X-ray lithography. X-rays have a particularly short wavelength and permit particularly fine features to be defined. In such lithography, a mask having the desired pattern thereon is interposed between the source of radiation, for example, UV or X-ray, and a semiconductor substrate coated with the resist on which the pattern is to be exposed. The mask must result in good definition of the lines being exposed, with the masked area being opaque to the radiation being used and the mask substrate being transparent to that radiation. Because of the small line widths which it is desired to achieve in X-ray lithography, the making of a mask which will provide good resolution becomes a problem.
The typical material used as an absorber on masks used in X-ray lithography is gold. Although other elements exhibit an increase in soft X-ray absorption, gold is notably easier to work with. Two methods that have been employed in the prior art are electroplating and evaporation/liftoff. Since certain of the mask substrates desired to be used in X-ray lithography are non-conductive, electroplating would be quite difficult. The evaporation/liftoff technique is described in co-pending application Ser. No. 107,744 now abandoned, filed 12-26-79, of D. W. Buckley and assigned to the same assignee as the present invention. It is a useful technique but has been found to be inapplicable for the manufacture of masks containing features smaller than one micron in size.
In a paper entitled "Electroless Gold Metallization for Polyvinylidene Fluoride Films", 125 Journal Electrochemical Society 522, April 1978, by L. M. Schiavone, there is described a technique for plating plastic film. As pointed out in that article, several important steps must be followed--namely, cleaning, sensitizing, activating, plating, and post cleaning. The author points out that cleaning is probably the most important step. After cleaning, the film is sensitized by immersing it in a bath of photosensitive salts. The film is then exposed. Thereafter, the film is immersed in a solution of a catalyst which will react with the activator chemicals. As pointed out by the author, palladium is an excellent catalyst for gold and he discloses a palladium chloride, acetic acid bath. After rinsing, the prepared film is immersed in a cyanide based gold plating bath where gold plates preferentially on the palladium catalyst. It has been observed that the gold deposited by this complex process adheres poorly to the substrate, an effect believed to be related to the thoroughness of the initial substrate cleaning.
Accordingly, it is an object of the present invention to provide a method for manufacturing a mask for X-ray lithography employing a substrate which may be non-conductive and forming thereon an X-ray absorbing gold pattern, the gold having an improved adherence to the substrate and being capable of defining features having dimensions of less than 1 micron. The manner in which this and other objects are achieved will be apparent from the following description and appended claims.